100 INVERTEBRATE ZOOLOGY 



the visceral mass. With forceps and knife carefully remove 

 the tough white integument which covers the left side of the 

 visceral mass. The soft cream-colored mass filling the greater 

 part of it is the reproductive gland, the greenish mass above is 

 the liver. Imbedded in these masses lies the alimentary tract, a 

 narrow, delicate tube, which will be injured in the dissection 

 unless the greatest care be taken. Beginning with the mouth 

 gently scrape away the soft mass which surrounds the alimen- 

 tary tract, laying it entirely bare. The water in the dissecting 

 pan must be frequently renewed to keep it clear, and great care 

 taken not to break the canal. The mouth opens into the short 

 oesophagus, after which the canal dilates to form the stomach. 

 The liver surrounds the stomach and is connected with it by 

 several ducts. Back of the stomach is the intestine, which first 

 runs backward and downward to the posterior part of the 

 visceral mass, after several turnings in the lower part of which 

 it bends upward and runs forward parallel with the posterior 

 margin of the visceral mass to its dorsal border, where it leaves 

 it. Here the rectum begins and passes through the heart and 

 above the posterior adductor muscle to the anus. A small 

 transparent rod is often present in the intestine; its function 

 is unknown. 



Clams feed upon minute organisms and organic particles 

 contained in the water. Some of the water in the mantle 

 cavity is drawn into the mouth by the ciliated oral palps and 

 passes through the alimentary tract, where the organic sub- 

 stances are digested and absorbed. 



Exercise 10. Draw a diagrammatic view of the digestive system. 



The reproductive system. The sexes are separate. The repro- 

 ductive glands (testis or ovary) are very similar to each other and 

 consist of a pair of cream-colored masses which fill a greater 

 part of the visceral mass. Their external openings are a pair 

 of minute pores, one on each side of the visceral mass just 



